Manufacture of two-way shape memory devices

ABSTRACT

A process is provided for the manufacture of a two-way shape memory alloy and device. The process of the invention allows a reversible adjustment of the characteristic transformation temperatures, as well as the direction of the two-way shape memory effect, at the final stage of manufacture.

This application is a Continuation-in-Part of U.S. patent applicationSer. No. 08/432,802, filed May 2, 1995, the contents of which areincorporated herein in their entirety now U.S. Pat. No. 5,624,508.

FIELD OF THE INVENTION

The present invention generally relates to shape memory alloys (SMA)i.e. alloys which can switch from one shape to another, "memorized"state upon a change in temperature. More specifically, the presentinvention relates to an SMA which is nickel-titanium based, also knownas Nitinol.

BACKGROUND OF THE INVENTION

Various metal alloys possess the ability to change their shape as aresult of a change in temperature. Such SMA can undergo a reversibletransformation from a martensitic state, in which the material isrelatively soft and deformable, to an austenitic state in which thematerial possesses super elastic properties and is relatively firm. Thetransformation from the martensitic state to the austenitic state willbe referred to herein as the "austenitic transformation", and the othertransformation, from the austenitic state to the martensitic state, willbe referred to herein as the "martensitic transformation". Theaustenitic transformation occurs over a range of temperature which ishigher than the range of temperatures in which the reversetransformation occurs. This means, that once transformed to anaustenitic state, an SMA will remain in that state even when cooled to atemperature below that in which the austenitic transformation began, aslong as the temperature is above that in which the martensitictransformation begins.

A particular class of SMAs are alloys of nickel and titanium-NiTialloys. NiTi alloys have found a variety of uses in medical as well asother fields. Medical uses of SMAs, particularly an NiTi-based alloy hasbeen described in U.S. Pat. Nos. 4,665,906, 5,067,957, European PatentApplication 143,580, U.S. Pat. No. 4,820,298 and many others.

For medical uses it is usually desired that the alloy will undergo anaustenitic transformation over a narrow, well defined range. Forexample, a vascular stent of the two-way SMA type, such as thatdescribed in European Patent Application, Publication No. 625153, istypically deployed in the body while being in the martensitic state atbody temperature, and then after heating, it transforms into theaustenitic state, and then remains in the austenitic state when cooledto the body temperature. It can be appreciated that if excessive heatingto transform the SMA from the martensitic to the austenitic state isrequired, this can be damaging to the surrounding tissue and is thusundesirable. Thus, it would ideally be desired that the austenitictransformation will begin at a temperature several degrees above bodytemperature and will be over a temperature range which will not causetissue damage owing to the excessive heating.

GENERAL DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a process for thetreatment of a NiTi-based alloy to obtain an alloy with a two-way shapememory effect (SME).

It is more specifically an object of the invention to provide such aprocess to obtain a two-way SME which does not require a multi-cycle"training" to yield a two-way SME.

It is still further an object of the invention to provide a process toobtain a two-way SME, with a narrow range of temperatures over which theaustenitic transformation occurs.

The process of the invention has two aspects. By one aspect, to bereferred to herein at times as the "said first aspect", the processyields an alloy with a direction of the austenitic and the martensitictransformations dictated by the direction of a conditioningtransformation in the martensitic state. In accordance with anotheraspect of the invention, to be referred to herein at times as the "saidsecond aspect", the process yields an alloy with a direction of amartensitic or austenitic transformations which is independent on thedeformation introduced in the martensitic state.

In the following description and claims the term "NiTi alloy" will beused to denote an alloy comprising primarily nickel and titanium atomsbut may contain also trace amounts of other metals). A NiTi alloy hastypically the following empiric formula:

    Ni.sub.l Ti.sub.m A.sub.n

wherein A represents Na, Cu, Fe, Cr or V

l, m, and n representing the proportions of the metal atoms within thealloy, the value of l, m and n being about as follows:

l=0.5

m=0.5-n

n=0.003 to 0.02.

In accordance with the invention, there is provided a process fortreating a raw NiTi alloy having an initial form to obtain an alloy witha final form in which it exhibits a two-way shape memory effect (SME)whereby it has an austcnitic and a martensitic memory state withassociated austenitic and martensitic shapes, respectively, the processcomprising the steps of:

(a) testing the raw NiTi alloy so as to obtain an estimate of thealloy's internal structure, by measuring the difference between A_(s)and A_(f) ;

(b) subjecting the raw NiTi alloy to a first heat treatment based onresults obtained in (a) so as to yield alloys with an initial internalstructure condition having essentially equal random dislocation density;

(c) subjecting the alloy to thermo-mechanical treatment (TMT),comprising plastic deformation of the alloy with simultaneous heating(e.g. by warm rolling or warm drawing) to yield, during dynamic ageingprocess (ageing while depressing), a polygonal sub-grain dislocationstructure decorated by precipitation;

(d) if the deformation in step (c) did not yield the final form,subjecting the alloy to an intermediate heat treatment to conclude onecycle of sub-grain dislocation structure formation;

(e) repeating steps (c) and (d) until yielding said final form; and

(f) subjecting the alloy to a final heat treatment and to a memorizingtreatment.

The TMT, while it may in some instance be performed in a single step,occasionally it is necessary to do it in a few steps if the total strainvalue could exceed a critical value which may give rise to formation ofpre-cracks (crack nucleation) in the alloy. The TMT is performed whilewarming the alloy typically to a temperature of about 0.3-0.6 Tm(Tm=melting temperature in °K.).

In accordance with one embodiment of the invention, the processcomprising the steps of:

(a) heating a sample of the raw NiTi alloy, to a temperature of about450°-550° C. for about 0.5-2.5 hours, and then testing the sample fortemperature difference between A_(s) and A_(f) ;

(b) subjecting the raw NiTi alloy to a first heat treatment based on theA_(f) -A_(s) difference obtained in step (a), as follows:

where the difference is less than about 7° C., heat treating the alloyto a temperature of about 450°-500° C. for about 0.5-1.0 hours;

where the difference is more than about 7° C., heat treating the alloyto a temperature of about 510°-550° C. for about 1.0-2.5 hours;

(c) subjecting the alloy to thermo-mechanical treatment, comprisingplastic deforming the alloy at a strain rate, of less than 5 sec⁻¹, withsimultaneous internal heating of a portion of the alloy where thedeformation occurs to a temperature of about 250°-550° C., thedeformation of this step being less than 55%, preferably less than 40%;

(d) if the deformation in step (c) did not yield the final form,subjecting the alloy to an intermediate heat treatment at a temperatureof about 500°-550° C., for about 0.5-2 hours, and then repeating step(c); and

(e) subjecting the alloy to a final heat treatment and to a memorizingtreatment.

The particulars of the final heat treatment and the memorizingtreatment, are different in said first aspect and in said second aspect.In accordance with said first aspect, this treatment comprises:

(i) forming the alloy into the form to be assumed by it in theaustenitic state,

(ii) subject the alloy to a polygonization heat treatment to yieldarrangement of random dislocation, then to solution treatment to releasenon arranged dislocation from precipitation and provide for theirrearrangement and then to an ageing treatment;

(iii) deforming the alloy to assume a conditioning form and treating itto memorize said austenitic state, which is the state into which it wasformed under (i) above, and a martensitic state, in which the alloy hasa martensitic form with an intermediate degree of deformation betweenthe austenitic form and the conditioning form.

Preferably, steps (ii) and (iii) of said first aspect, comprise:

(ii) subjecting the alloy to a polygonization heat treatment at about450°-550° C. for about 0.5-1.5 hours, then to solution treatment atabout 600°-800° C. for about 2-50 mins., and then to an ageing treatmentat about 350°-500° for about 0.15-2.5 hours, and

(iii) deforming the alloy to assume a conditioning form, the deformationbeing less than about 15%, and preferably less than 7%, and beingperformed at a temperature T, which meets the following formula

    T<M.sub.s +30° C.

wherein M_(s) is a temperature where the martensitic transformationbegins, and then heating the alloy to a temperature of or above that inwhich the austenitic transformation of the alloy ends.

It should be pointed out that although a single cycle of deformation instep (iii) above is usually sufficient, it may at times be desired torepeat this cycle one or more times.

In accordance with said second aspect, the final heat and memorizingtreatment comprises:

(i) forming the alloy into a form other than the form to be assumed byit in the austenitic state,

(ii) subjecting the alloy first to heat treatment, then topolygonization and solution treatment and then optionally to ageingtreatment;

(iii) forming the alloy into a form to be assumed by it in theaustenitic state,

(iv) subjecting the alloy to a memorizing heat treatment and to anageing treatment; whereby the alloy is conditioned to memorize anaustenitic state in which it has an austenitic form assumed by it in(iii) above, and a martensitic state, wherein it has a martensitic formbeing a form with an intermediate degree of deformation between the formin which the alloy was formed in (i) above and the austenitic form.

By a preferred embodiment of said aspect, steps (ii) and (iv), comprise:

(ii) subjecting the alloy to a heat treatment at about 450°-500° C. forabout 0.5-2 hours, then subjecting the alloy to polygonization andsolution treatment at about 600°-800° C. for about 2-50 mins., and thensubjecting the alloy to aging treatment at about 350°-500° C. for about0-2 hours,

(iv) subjecting the alloy to a memorizing heat treatment at about500°-600° C. for more than about 10 mins., and then subjecting the alloyto ageing treatment at about 350°-500° C. for about 0.15-2.5 hours.

Following the treatment in accordance with both said first and saidsecond embodiments, A_(f) will be between about 10° to about 60° C. Inorder to increase A_(f) and A_(s), the alloy may then be subjected toageing heat treatment at a temperature of about 350°-500° C. In order todecrease A_(f) and A_(s), the alloy can then be subjected to a solutiontreatment at a temperature of about 510° to about 800° C.

By differential ageing or solution treatment in different portions thealloy will have different temperatures of austenitic transformation.This is at times desired, for example, in the case of a medical stent,to have portions thereof with different transition temperatures ofaustenitic transformation and/or martensitic transformation.

By the above process, SMAs for a variety of applications may beprepared. Examples are medical devices, e.g. various orthopaedicdevices, tooth rooth implants, medical stents, intrauterine implants, aswell as non-medical devices, e.g. tube joints. A process for thepreparation of such medical devices, as well as devices prepared by suchprocess also form an aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 in the drawings shows the relation between A_(f) and the ageingtime, in different ageing temperatures.

DETAILED DESCRIPTION OF THE INVENTION

The temperature range over which the austenitic transformation takesplace, is critical in a variety of medical applications. A specific casein point are medical stents such as those made of a two-way shape memoryalloy (SMA) described in European Patent Application No. 626153. Such ashape memory (SM) device, is deployed in a tubular organ at bodytemperature, and then heated so as to allow the occurrence of theaustenitic transformation. Once heated, it remains in the austeniticstate in body temperature and supports the wall of the tubular organ.Such SM devices are designed so that the beginning of the austenitictransformation will occur at a temperature at or above 40° C. It willhowever be appreciated, that the range of temperature over which theausteritic transformation occurs should desirably be narrow sinceexcessive heating where the temperature range is large, can cause tissuedamage. Furthermore, a narrow temperature range will generally ensurealso a more rapid transition from the martensitic to the austeniticstates.

In the following description the invention will be described at timeswith particular reference to its application for the preparation ofmedical SM devices with a narrow range of austenitic transformation. Itwill however, be appreciated, that the invention is not limited theretoand the application of the invention to the preparation of medicalstents is exemplary only. In accordance with the present invention, araw Niti alloy, which is typically provided by manufacturers in the formof a wire or a rod is first tested for the difference between A_(s) andA_(f). For this purpose a small sample of the material is taken. Basedon the A_(s) -A_(f) difference, the alloy, e.g. the wire or the rod, isthen subjected to a first heat treatment.

Following the first heat treatment, the alloy is subjected to athermo-mechanical treatment (TMT) where the alloy is simultaneouslyheated and subjected to a mechanical deformation. In the case of aprocess intended for the manufacture of a medical SM device, themechanical deformation typically involves changing the form of thealloy, from an initial form of a wire or rod, to that of a ribbon, band,etc; or alternatively, changing the wire or rod into a wire or rod of asmaller diameter. In order to retain the shape memory effect (SME) ofthe alloy, the total degree of deformation during the TMT, should beless than 55%, preferably less than 40%. Where the total requiredeventual deformation is more than 55%, the TMT is performed in two stepswith an intermediate heat treatment.

The thermo-mechanical treatment may, for example, be: warm roling wherethe alloy is processed to be used as a medical stent; warm drawing wherethe alloy is processed to be used as an orthopaedic tooth root implant;etc. In warm rolling or drawing the alloy is typically heated to atemperature of about 0.3-0.6 Tm (Tm being the melting temperature in°K.). The heating of the deforming portion must be byelectro-stimulation, e.g. at a current density of about 500-2000 A/cm².A big advantage of such a treatment is that in addition to causing amechanical deformation, it leads also to heating of the pre-cracks witha high dislocation density owing to the relatively high electricalresistance at such pre-cracks which gives rise to a selectiveoverheating at such points and heating of the pre-cracks. Moreover,clectro-stimulated warn TMT at the above current density acceleratesdislocation reaction, which results in a perfect dislocation subgrainstructure formation. Furthermore, the heating electrical current givesrise to a dynamic ageing process with a second phase precipitation onthe walls of the subgrain dislocation cells. This structure provides fora very narrow thermal interval of austenitic transformation A_(f) -A_(s)for the shape memory alloy, and for a variety of other advantageousproperties to be explained below.

In an electrically stimulated warm rolling, where the current densitydecreases below 500 A/cm², or the strain rate of the deformation isabove about 5 sec⁻¹, there is an increase of the random dislocationdensity which will decrease the degree of perfection of the subgrainstructure. For a narrow A_(f) -A_(s) intervals a subgrain structure asperfect as possible is required. Accordingly, with an increase in therandom dislocation density there is an increase in the A_(f) -A_(s)interval. For example, where the current density is about 400 A/cm², orwhore strain rate is about 8 sec⁻¹, the A_(f) -A_(s) interval afterfinal heat treatment will be about 10°-12° C. Furthermore, increasing ofthe current density to above about 2000 A/cm², leads to arecrystallization process, that prevents formation of the necessarysubgrain cells with precipitation on the cell walls.

The memorizing treatment involves a conditioning step in whichmicroscopic changes within the alloy condition it to "memorize" the twoforms which the alloy assumes during its use, that in the martensiticstate ("martensitic form") and that in the austenitic state ("austenticform").

In accordance with said first aspect, the alloy is formed into a shapeto be assumed by it in the austenitic state, e.g. in the case of a stentthis involves winding on a mandrel having a diameter of a stent in theaustenitic state. The alloy is then typically placed in a vacuum orinert atmosphere furnace, in which it is first subjected to a memorizingand internal structure polygonization treatment, at a temperature ofabout 450°-550° C. for about 0.5-1.5 hours, and then heated to about600°-800° C. for about 2-50 mins. During this latter heating, the alloyundergoes a solution treatment with re-arrangement of dislocations whichare freed after solution treatment. Subsequently, the alloy is subjectedto a final aging treatment at a temperature of about 350°-500° C. forabout 0.15-2.5 hours.

The result of the above treatment is a subgrain structure which impartsthe alloy with several features. For one, the temperature of theaustenitic transformation, A_(f), can be adjusted within a range of10°-60° C. with a very narrow interval of A_(f) -A_(s) of about 1°-5° C.

In case it is desired to decrease A_(f), the alloy may be subjected to asolution treatment at a temperature of about 510°-800° C. In order toachieve a desired A_(f), both the temperature as well as the ageing timecan be controlled. For example, where the Nitinol alloy has after thefinal heat treatment an A_(s) of about 45° C. and an A_(f) of about 48°C., after a solution treatment at 640° C. for about 5 mins., the A_(s)and A_(f) decrease to about 23° C. and 27° C., respectively; followingsolution treatment at 640° C. for 10 mins., A_(s) and A_(f) decrease toabout 11° C. and 15° C., respectively.

In order to increase A_(f), the alloy is subjected to an ageing heattreatment at a temperature of about 350°-500° C. Here again, in order toachieve a desired A_(f), both the temperature as well as the ageing timecan be controlled. This is demonstrated, for example, in FIG. 1 whichgives the relation between the ageing time at two different temperatures(380° C. and 480° C.) and the resulting A_(f), following a solutiontreatment at 640° C. for 20 mins. As can be seen, for example, ageingtreatment at 380° C. for about 100 mins. yields an A_(f) of about 40°C., with the same A_(f) being reached with an ageing treatment at 480°C. of about 40 mins. Ageing at a temperature of about 450° C. for about80 mins. will yield an A_(s) of about 46° C. and an A_(f) of about 49°C. (not shown in FIG. 1).

A unique feature of the process of the invention is the fact that thetwo-way SME is induced by only one cycle of deformation. In the case ofsaid first aspect of the invention, this may be achieved by deformingthe alloy into a conditioning form at a temperature of T<M_(s) +30° C.followed by heating to a temperature at or above the A_(f) of the alloy.The deformation should be less than 15% and preferably less than 7%. Adeformation above 15% will effect the internal structure of the materialand yield a total or partial loss of the memory form of the austeniticstate. A deformation between 7% and 15% will have only such a partialharming effect. The martensitic memory form which the alloy assumesafter the above conditioning step, is an intermediate form between theaustenitic memory form and the conditioning form. The direction of thetwo-way SME following such memorizing treatment, coincides with thedirection in the martcnsitic state deformation. For example, where adeformation in the martensitic state involves a decrease in diameter,the diameter of the alloy in the martensitic state will be less thanthat of the austenitic state, and vice versa.

Generally, the process in accordance with said first aspect allows areversible adjustment of the characteristic transformation temperaturesas well as the direction of the two-way SME at the final stage ofmanufacture.

A final memorizing treatment in accordance with the second aspect of theinvention, gives rise to a two-way SME without the need for a finaldeformation to induce the two-way SME. This effect is not determinedwhen indirect SME occurs. The second aspect is useful, for example, forthe manufacture of a stent with a two-way SME, and the description belowwill refer to this specific embodiment, The Niti ribbon or wire is woundon a mandrel having a diameter equal to 2R₁ constrained and placed intoa vacuum furnace, at a temperature of about 450°-550° C. for about0.5-1.5 hours, so that internal structure normalization and texturalformation takes place. Similarly as above, the alloy is then subjectedto solution treatment and structure improvement at a temperature of600°-800° C. for 2-50 mins. and then to an ageing treatment at 350°-500°C. for 0.15-2.5 hours. The ribbon or wire is then rewound on a mandrelwith a diameter 2R₂, which is the diameter to be assumed by the stent inthe austenitic state and then subjected to a memorizing and ageingtreatment at temperature of 450°-550° C. for 0.15-2 hours. If the strainof this treatment ε_(treat) =1/2 w(1/R₂ -1/R₁)<0 (w being the thicknessin case of a ribbon and the diameter in case of a wire) thecorresponding strain of the two-way SME during cooling ε_(tw) =1/2w(1/R_(tw) -1/R₂)>0 (R_(tw) being the diameter of the stent whenassuming its martensitic state) and vice versa. As a result of thistreatment, there is a very narrow temperature range in which theaustenitic transformation takes place, A_(f) -A_(s) =1°-5° C., with apossibility to change A_(f) between 10° and 60° C., similarly as above,

The two-way SME in cooling may either coincide or oppose the directionof the deformation in the martensitic state. In case R₂ is larger thanR₁, and R_(tw) will be smaller than R₂, the device shrinks when it iscooled. In case R₂ is less than 0, i.e. a reverse bending, and R₂ islarger than R_(tw), the device will expand when cooled.

Finally, another result of the process of the present invention is ahigh resistance of the formed alloy, to pitting corrosion and hydrogenembrittlement which may occur in the biological media with theirrelatively high chlorine ion content.

The invention will now be illustrated further by several specificexamples.

EXAMPLE 1

Preparation of a Biliary Stent

The starting material was a super-elastic Niti wire, with a diameter of1.5 mm. The Ti and Ni content of the alloy was 50 to 50.8 at % (at %=%atoms out of the total number of atoms in the alloy) and 49.1 at %,respectively. A sample of the wire was treated at a temperature of 500°C. for 1.5 hours and the temperature interval A_(f) -A_(s) wasdetermined and was found to be 15° C.

The wire was then subjected to a first heat treatment at 550° C. for twohours, and then to a thermo-mechanical electro-simulated treatment, withthe current density being 900 A/cm² and the strain rate being 0.3 sec⁻¹.The TMT was repeated three times with two intermediate heat treatmentsat 500° C. for one hour each. The ribbon thickness was eventuallyreduced to 0.25 mm.

The ribbon was then wound and constrained on a mandrel having a diameterof 8 mm, and placed into a vacuum furnace and heated to 500° C. for 0.6hours, and then subjected to a solution treatment at 650° C. for 30mins. This was followed by an aging treatment at 400° C. for 1 hour.

The spiral coiled stent which was obtained had an A_(s) of 40° C. and anA_(f) of 43° C.

The stent was then wound on a 3 mm. diameter mandrel at a temperature of25° C. and heated to above 43° C. for shape recovery. Thus, a stent witha two-way SME was obtained, having an austenitic memory form in whichits diameter was 8 mm, a martensitic memory form to which it shrank whencooled below 25° C. in which it had a diameter of 7.3 mm.

In order to install the stent, in situ within the body, it is wound on acatheter, and then inserted into the desired place within the bile duct.The stent is then activated by raising its temperature to more than 43°C. To remove the stent it has to be cooled below 25° C. and aftershrinking it can be pulled away.

EXAMPLE 2

Esophageal Stent

A stent was prepared from the same TiNi wire as used in example 1. Thewire was subjected to a first heat treatment and then to a TMT,similarly as described in Example 1, the difference being that the finalthickness of the wire which was obtained was 0.28 mm.

The ribbon was then wound on a mandrel having a diameter of 70 mm, wasconstrained and then heated to 500° C. for 1 hour and then to a solutiontreatment at 650° C. for 20 mins. The ribbon was then wound on a mandrelhaving a diameter of 16 mm., was constrained and subjected to amemorizing treatment at 520° C. for 30 mins., and then to agingtreatment at 400° C. for 2 hours, The stent which was obtained afterthis procedure had the following parameters: A_(s) =42° C.; A_(f) =45°C.; temperature of martensitic transformation being 27° C., with thestent expanding when cooling from a diameter of 16 mm. which it had inthe austeitic state to a diameter of 18 mm. in the martensitic state.

For deployment, the stent is wound on a catheter with a diameter of 5mm, inserted into the desired place within the esophageal tract and isactivated by heating above 45° C. When the stent is cooled, it expandswhich prevents the stent from falling into the stomach.

EXAMPLE 3

Esophageal Stent

A stent was prepared in a similar manner as that of Example 2, with thedifference being that the ribbon was wound on a mandrel having adiameter of 5 mm. and after heat treatment was rewound on the mandrelwith the opposite direction. After heat treatment, similarly as inExample 2, the stent expands when cooled from a diameter of 16 mm. to adiameter of 25 mm.

EXAMPLE 4

Shape Memory Force Element for Orthopedic Compression Screw

Starting material was 1.5 mm diameter NiTi wire (the composition of thealloy was 50.5 at % Ni and 49.5 at % Ti). The wire was subjected to afirst heat treatment and then to a TMT, similarly as described inExample 1 (however using warm drawing instead of warm rolling).

The wire was then heat treated in straight constrained condition under atemperature of 500° C. for 0.5 hours and then subjected to a solutiontreatment at 650° C. for 20 mins. The wire was then released andsubjected to a memorizing treatment at 520° C. for 30 mins., which wasfollowed by an ageing treatment at 450° C. for 1 hour. After wireelongation from 20 to 21 mm, shape memory effect was obtained with A_(s)=39° C. and A_(f) =41° C. and after cooling up to 25° C. two way shapememory effect occurs just after heat treatment (without training) andincreases after the training process (stretching-heating).

EXAMPLE 5

Shape Memory Medical Staples

Starting material and treatment was similar to that of Example 4. Thefinal diameter which was achieved (by warm drawing) was 0.25 mm. Thewire was constrained in the necessary shape and heat treated after TMTat 520° C. for 0.5 hours, solution treatment at 680° C. for 10 mins. andageing at 450° C. for 1.5 hours.

After staple bending, shape memory effect was obtained with A_(s) =42°C. and A_(f) 45° C.

EXAMPLE 6

Tooth Root Implants

The starting material was a super-elastic Nitinol (50.8 at % Ni) rodwith diameter of 10 mm. The rod was subjected to a first het treatmentat 550° C. for 2 hours and then to a TMT--drawing at 500° C., with astrain rate of 0.5 sec⁻¹. The TMT was repeated 2 times with intermediateheat treatment at 500° C. for 1 hour. The rod had a final diameter of6.0 mm.

The rod was machined into a shape of a tooth root implant with 6 forcesegments (legs) for anchoring into the jaw bone. The leg's length wasd3, 4 and 5 mm for different implants. The implant was then subjected topoligonization heat treatment at 500° C., for 1 hour, then the implant'slegs were distorted on the mandrel, following which the implant wassubjected to heat treatment at 650° C., for 30 min and to ageing at 480°C. for 1.5 hour. Then the implant's legs were forced together with aconic cup, (from distorted diameter 5.0 mm to closed conditions withdiameter 3.0 mm). While heating the implant, it causes the legs to openat temperatures: A_(s) =38° C. and A_(f) =42° C., that yields verygentle pressure on the jaw bone and very safe implant activation. Asingle cycle of straightening of the implant's legs and subsequentheating induces two way SME in the direction of joining of the legs withcooling, which feature is useful for removing the implant.

EXAMPLE 7

Two way SM tube coupling with narrow A_(s) -A_(f) interval

A 10 mm NiTi rod identical to that which served as the starting materialin Example 6 was treated in the same manner as described in Example 6 toyield a rod with a diameter of 6 mm. This rod was then machined into theshape of the hollow cylinder with an internal diameter (ID) of 4.4 mm.Then the cylinder was subjected to poligonisation and solution heattreatment: 500° C. for 1 hour and then of 680° C. for 20 min. It wasthen cooled, expended on a mandrel with diameter of 4.5 mm and subjectedto memorising and ageing heat treatment: 530° C. for 30 min and 430° C.for 40 min. The tube coupling was then cooled and expanded on themandrel to yield an internal diameter of 4.75 mm.

The coupling joins tubes after heating (A_(s) =15° C., A_(f) =18° C.)and it has two way SME in direction of reduction of ID. Thus, even withcooling it keeps pressure on the joined tubes. In comparison,conventional couplings where the two way SME is induced duringinstallation (expansion and heating) in direction of expansion, coolingresults in loosening of the coupling.

We claim:
 1. A process for treating a raw NiTi alloy having an initialform to obtain an alloy with a final form in which it exhibits a two-wayshape memory effect (SME) whereby it has an austenitic and a martensiticmemory state with associated austenitic and martensitic shapes,respectively, the process comprising the steps of:(a) testing the rawNiTi-based alloy so as to obtain an estimate of the alloy's internalstructure, by measuring the difference between A_(s) and A_(f), whereinA_(s) is a temperature where austenitic transformation, namelytransformation between the martensitic to the austenitic state, begins,and A_(f) is a temperature where the austenitic transformation ends; (b)subjecting the raw NiTi alloy to a first heat treatment at a temperatureand for a time determined on the basis of the A_(f) -A_(s) differenceobtained in step (a) so as to yield alloys with an initial internalstructure condition having essentially equal random dislocation density;(c) subjecting the alloy to thermo-mechanical treatment (TMT),comprising plastic deformation of the alloy with simultaneous heatingduring a dynamic ageing process, to yield a polygonal sub-graindislocation structure containing precipitates; (d) if the deformation instep (c) did not yield the final form, subjecting the alloy to anintermediate heat treatment to conclude one cycle of sub-graindislocation structure formation; (e) repeating steps (c) and (d) untilyielding said final form; and (f) subjecting the alloy to a final heattreatment and to a memorizing treatment.
 2. A process according to claim1, comprising the steps of:(a) heating a sample of the raw NiTi alloy,at a temperature of about 450°-550° C. for about 0.5-2.5 hours, and thentesting the sample for temperature difference between A_(s) and A_(f) ;(b) subjecting the raw NiTi alloy to a first heat treatment based on theA_(f) -A_(s) difference obtained in step (a), as follows:where thedifference is less than about 7° C., heat treating the alloy at atemperature of about 450°-500° C. for about 0.5-1.0 hours; where thedifference is more than about 7° C., heat treating the alloy at atemperature of about 510°-550° C. for about 1.0-2.5 hours; (c)subjecting the alloy to TMT, comprising plastic deforming the alloy at astrain rate, of less than 5 sec⁻¹, with simultaneous internal heating ofa portion of the alloy where the deformation occurs at a temperature ofabout 250°-550° C., the deformation of this step being less than 55%;(d) if the deformation in step (c) did not yield the final form,subjecting the alloy to an intermediate heat treatment at a temperatureof about 500°-550° C., for about 0.5-2 hours, and then repeating step(c); and (e) subjecting the alloy to a final heat treatment and to amemorizing treatment.
 3. A process according to claim 2, wherein thedeformation in step (c) is less than 40%.
 4. A process according toclaim 1, wherein the final heat and memorizing treatment comprises:(i)forming the alloy into the form to be assumed by it in the austeniticstate, (ii) subject the alloy to a polygonization heat treatment toyield arrangement of random dislocation, then to solution treatment torelease non arranged dislocation from precipitation and provide fortheir rearrangement and then to an ageing treatment; (iii) deforming thealloy to assume a conditioning form and treating it to memorize saidaustenitic state, which is the state into which it was formed under (i)above, and a martensitic state, in which the alloy has a martensiticform with an intermediate degree of deformation between the austeniticform and the conditioning form.
 5. A process according to claim 4,wherein the final heat and memorizing treatment comprises:(i) formingthe alloy into the form to be assumed by it in the austenitic state;(ii) subjecting the alloy to a polygonization heat treatment at about450°-550° C. for about 0.5-1.5 hours, then to solution treatment atabout 600°-800° C. for about 2-50 mins., and then to an aging treatmentat about 350°-500° C. for about 0.15-2.5 hours, and (iii) deforming thealloy to assume a conditioning form, the deformation being less thanabout 15%, and being performed at a temperature T, which meets thefollowing formula

    T<M.sub.s +30° C.

wherein M_(s), is a temperature where the martensitic transformationbegins, and then heating the alloy to a temperature of or above that inwhich the austenitic transformation of the alloy ends.
 6. A processaccording to claim 5, wherein the deformation of the alloy to assume theconditioning form in step (e) (iii), is less than about 7%.
 7. A processaccording to claim 5, comprising:(a) adjusting the temperature in whichthe austenitic transformation occurs, by eitheran aging treatment at atemperature of about 350°-500° C., to increase the temperature in whichthe austenitic transformation occurs, or a solution treatment at atemperature of about 510°-800° C., to decrease the temperature in whichthe austenitic transformation occurs.
 8. A process according to claim 5,wherein the simultaneous internal heating in step (c) compriseselectro-stimulation with a current density of about 500-2000 A/cm².
 9. Aprocess according to claim 1, wherein the final heat and memorizingtreatment comprises:(i) forming the alloy into a form other than theform to be assumed by it in the austenitic state, (ii) subjecting thealloy first to heat treatment, then to polygonization and solutiontreatment and then optionally to ageing treatment; (iii) forming thealloy into a form to be assumed by it in the austenitic state, (iv)subjecting the alloy to a memorizing heat treatment and to an ageingtreatment;whereby the alloy is conditioned to memorize an austeniticstate in which it has an austenitic form assumed by it in (iii) above,and a martensitic state, wherein it has a martensitic form being a formwith an intermediate degree of deformation between the form in which thealloy was formed in (i) above and the austenitic form.
 10. A processaccording to claim 9, wherein the final heat and memorizing treatmentcomprises:(i) forming the alloy into a form other than the form to beassumed by it in the austenitic state, (ii) subjecting the alloy to aheat treatment at about 450°-500° C. for about 0.5-2 hours, thensubjecting the alloy to polygonization and solution treatment at about600°-800° C. for about 2-50 mins., and then subjecting the alloy toaging treatment at about 350°-500° C. for about 0-2 hours, (iii) formingthe alloy into a form to be assumed by it in the austenitic state, and(iv) subjecting the alloy to a memorizing heat treatment at about500°-600° C. for more than about 10 mins., and then subjecting the alloyto aging treatment at about 350°-500° C. for about 0.15-2.5 hours.
 11. Aprocess for preparing a medical device comprising a shape memory alloy(SMA) embodying a two-way shape memory effect, comprising treating theSMA in accordance with the process defined in claim
 1. 12. A processaccording to claim 11, wherein said medical device is a stent.
 13. Aprocess for the manufacture of a medical stent from a NiTi alloy, beinga wire having a first diameter, the stent having either the form of awire with a second diameter or a form of a band, the stent exhibiting atwo-way shape memory effect (SME) having an austenitic and a martensiticmemory state with associated austenitic and martensitic shapes,respectively, the process comprising the steps of:(a) heating a sampleof the NiTi wire at a temperature of about 450°-550° C. for about0.5-2.5 hours, and then testing the sample for temperature differencebetween A_(s) and A_(f), where A_(s) is a temperature wherein austenitictransformation, namely transformation between the martensitic to theaustenitic state, begins, and A_(f) is a temperature where theaustenitic transformation ends; (b) subjecting the wire to a first heattreatment based on the A_(f) -A_(s) difference obtained in step (a), asfollows:where the difference is less than about 7° C., heat treating thewire at a temperature of about 450°-500° C. for about 0.5-1.0 hours;where the difference is more than about 7° C., heat treating the wire ata temperature of about 510°-550° C. for about 1.0-2.5 hours; (c)subjecting the wire to a thermo-mechanical treatment, comprising warmrolling of the wire at a strain rate of less than 5 sec⁻¹, withsimultaneous internal heating of a portion of the wire where thedeformation occurs, the heating by electro-stimulation at a currentdensity of about 500-2000 A/cm², the deformation in this step being lessthan 55%; (d) where the deformation in step (c) did not yield across-sectional shape of the final form, subjecting wire to anintermediate heat treatment at a temperature of about 500°-550° C., forabout 0.5-2 hours and then repeating step (c); and (e) subjecting thewire to a final heat treatment and to a memorizing treatment, whichcomprises:(i) winding the wire or band obtained in step (c) on a mandrelhaving a diameter to be assumed by the stent in the austenitic state,(ii) subjecting the wire to a polygonization heat treatment at about450°-550° C. for about 0.5-1.5 hours, then to solution treatment atabout 600°-800° C. for about 2-50 mins., and then to an aging treatmentat about 350°-500° C. for about 0.15-205 hours, (iii) deforming the wireby winding it on a mandrel having a conditioning diameter, thedeformation being less than about 7%, and being performed at atemperature T, which meets the following formula

    T<M.sub.s +30° C.

wherein M_(s) is a temperature where the martensitic transformationbegins, and then heating the wire or band to a temperature at or abovethat in which the austenitic transformation ends;whereby a stent isobtained with an austenitic state in which it has a diameter assumed in(i) above and a martensitic state in which it has a diameter which is anintermediate diameter between the conditioning diameter and theaustenitic diameter.
 14. A process for the manufacture of a medicalstent from a wire made of a NiTi alloy having a first diameter, thestent having either the form of a wire with a second diameter or a formof a band, the stent exhibiting a two-way shape memory effect (SME)having an austenitic and a martensitic memory state with associatedaustenitic and martensitic shapes, respectively, the process comprisingthe steps of:(a) heating a sample of the wire at a temperature of about450°-550° C. for about 0.5-2.5 hours, and then testing the sample fortemperature difference between A_(s) and A_(f) wherein A_(s), is atemperature wherein austenitic transformation, namely transformationbetween the martensitic to the austenitic state, begins, and A_(f) is atemperature where the austenitic transformation ends; (b) subjecting thewire to a first heat treatment based on the A_(f) -A_(s) differenceobtained in step (a), as follows:where the difference is less than about7° C., heat treating the wire at a temperature of about 450°-500C. forabout 0.5-1.0 hours; where the difference is more than about 7° C., heattreating the wire at a temperature of about 510°-550° C. for about1.0-2.5 hours; (c) subjecting the wire to a thermo-mechanical treatment,comprising warm roiling of the wire at a strain rate of less than 5sec⁻¹, with simultaneous internal heating of a portion of the wire wherethe deformation occurs, by electro-stimulation at a current density ofabout 500-2000 A/cm², the deformation in this step being less than 55%;(d) where the deformation in step (c) did not yield a cross-sectionalshape of the final form, subjecting wire to an intermediate heattreatment at a temperature of about 500°-550° C., for about 0-2 hoursand then repeating step (c); and (e) subjecting the wire to a final heattreatment and to a memorizing treatment, which comprises:(i) winding thewire or band obtained in step (c) on a mandrel having a conditioningdiameter being different than the diameter to be assumed by the stent inthe austenitic state, (ii) subjecting the wire to a heat treatment atabout 450°-500° C. for about 0.5-2 hours, then to polygonization andsolution treatment about 600°-800° C. for about 2-50 mins., and then toaging treatment at about 350°-300° C. for about 0-2 hours, (iii) windingthe wire or band on a mandrel having a diameter to be assumed by thestent in the austenitic state, (iv) subjecting the alloy to a memorizingheat treatment at about 500°-600° C. for more than about 10 mins., andthen to aging treatment at about 350°-500° C. for about 0.15-2.15 hours;whereby a stent is obtained having an austenitic state with a diameterinto which the wire was formed in step (iii), and a martensitic state inwhich the stent has a diameter which is an intermediate diameter betweenthe conditioning diameter and the diameter of the stent in theaustenitic state.
 15. A process for the manufacture of tooth rootimplant from a NiTi alloy, exhibiting a two way SME having an austeniticand a martensitic memory state with associated austenitic andmartensitic shapes, respectively, the process comprising the stepsof:(a) heating a sample of a NiTi rod at a temperature of about450°-550° C. for about 0.5-2.5 hours, and then testing the sample fortemperature difference between A_(s) and A_(f), wherein A_(s) is atemperature wherein austenitic transformation, namely transformationbetween the martensitic to the austenitic state, begins, and A_(f) is atemperature where the austenitic transformation ends; (b) subjecting therod to a first heat treatment based on the A_(f) -A_(s) differenceobtained in step (a), as follows:where the difference is less than about7° C., heat treating the wire at a temperature of about 450°-500° C. forabout 0.5-1.0 hours; where the difference is more than about 7° C., heattreating the wire at a temperature of about 510°-550° C. for about1.0-2.5 hours (c) subjecting the rod to a TMT comprising warm drawingwith a strain rate less than 5 sec⁻¹ with simultaneous heating, thetotal strain in this step being less than 55%; (d) where the deformationin step (c) did not yield a cross-sectional shape of the final form,subjecting the rod to an intermediate heat treatment at a temperature ofabout 500°-550° C., for about 0.5-2 hours and then repeating step (c);(e) machining the rod to yield the shape of the implant; (f) subjectingthe implant to the final heat treatment and to a memorizing treatment,which comprises:(i) expanding implant's force segments to a diameter tobe assumed by the implant in the austenitic state, (ii) subjecting theimplant to a polygonization heat treatment at about 450°-550° C. forabout 0.5-1.5 hours, then to solution treatment at about 600°-800° C.for about 2-50 min, and then to an ageing treatment at about 350°-500°C. for about 0.15-2.5 hours, (iii) deforming the implant force segmentsto a conditioning diameter with a strain less than about 7% and beingperformed at temperature T<M_(s) +30° C., wherein M_(s) is a temperaturewhere the martensitic transformation begins and then heating the implantto a temperature at or above that in which the austenitic transformationends; whereby an implant is obtained with an austenitic state in whichit has a diameter assumed in (i) above and a martensitic state in whichit has a diameter, which is an intermediate diameter between theconditioning diameter and the austenitic diameter.
 16. A process for themanufacture of tube coupling from a NiTi alloy, exhibiting a two way SMEhaving an austenitic and a martensitic memory state with associatedaustenitic and martensitic shapes, respectively, the process comprisingthe steps of:(a) heating a sample of a NiTi rod at a temperature ofabout 450°-550° C. for about 0.5-2.5 hours, and then testing the samplefor temperature difference between A_(s) and A_(f) wherein A_(s), is atemperature wherein austenitic transformation, namely transformationbetween the martensitic to the austenitic state, begins, and A_(f) is atemperature where the austenitic transformation ends; (b) subjecting therod to a first heat treatment based on the A_(f) -A_(s), differenceobtained in step (a), as follows:where the difference is less than about7° C., heat treating the wire at a temperature of about 450°-500° C. forabout 0.5-1.0 hours; where the difference is more than about 7° C., heattreating the wire at a temperature of about 510°-550° C. for about1.0-2.5 hours; (c) subjecting the rod to a TMT comprising warm drawingwith a strain rate less than 5 sec⁻¹ with simultaneous heating, thetotal strain in this step being less than 55%; (d) where the deformationin step (c) did not yield a cross-sectional shape of the final form,subjecting the rod to an intermediate heat treatment at a temperature ofabout 500°-550° C., for about 0.5-2 hours and then repeating step (c);and (e) machining the rod to yield the shape of the implant; (f)subjecting the implant to the final heat treatment and to a memorizingtreatment, which comprises:(i) subjecting the coupling to apolygonization heat treatment at about 450°-550° C. for about 0.5-1.5hours, then to solution treatment at about 600°-800° C. for about 2-50min and then to an ageing treatment at about 350°-500° C. for about0-2.5 hours, (ii) expanding the coupling to a diameter to be assumed bythe coupling in the austenitic state, (iii) subjecting the coupling to amemorizing heat treatment at about 500°-600° C. for more than about 10mins, and then to an ageing treatment at about 350°-500° C. for about0.15-2.5 hours, whereby a coupling is obtained having an austeniticstate with a diameter into which it was formed in step (ii) and amartensitic state in which the coupling has a diameter which isintermediate diameter between the conditioning ID and the diameter ofthe coupling in the austenitic state.